I make a Buttermilk Alfredo sauce that goes great in pasta or mashed taters! Add a couple of tablespoons of cornstarch to a quart of buttermilk. Leave that to sit while you sautee onion, mushrooms, and spinach. Pour in the buttermilk and season with salt, pepper, and any herbs you want. Stir until it bubbles (keep it on medium heat), then add a melty cheese. Serve on pasta or mix into mashed potatoes.

Last time I had leftover buttermilk I made an "oven fried" chicken recipe that turned out great. Marinated the chicken pieces overnight in buttermilk, garlic, a little chopped up chipotle, honey and thyme (+ salt & pepp). Then breaded and baked. Deelish.

If you search 'buttermilk' in recipes, you'll find 30+ pages -- I just breezed through the first 5 pages and I'm excited. One that particularly appeals to me is the Buttermilk Pie with chilis! Here's one that I posted, which seems to me to get better every time I make it: http://www.food52.com/recipes.... Also scones, breads from AntoniaJames, and so on...
Once I started keeping buttermilk in the fridge, I've found that I love it. Tangy taste, easy to pour and measure and it keeps for a long time. You can use it in place of yogurt, making adjustment if needed.

Great for making homemade dips. Add to cream cheese, put in some dill or dehydrated onion, parsely, etc. If you have no cream cheese sour cream works too or just use the buttermilk alone. It will be more runny but the kids won't notice if you cut up some carrots, celery etc and put it on a plate.

Merrill's Saag Paneer (and you can make your own paneer while you're at it, using buttermilk, with Jennifer Perillo's Creamy Homemade Ricotta . . . just strain it longer, while pressing out the liquid, to make cheese. Don't forget to save the whey, for use in polenta, breads, creamy soups, etc.). I agree with everyone else's suggestions. Wonderful in cream of spinach soup; so, so easy, especially if you use frozen spinach. Also, keep in mind that buttermilk usually holds well beyond its "sell-by" date if kept in the back of the fridge. ;o)